The plort market is a network of buyers and sellers of plorts across the galaxy. Ranchers use plort market machines on their ranch to send plorts to the market for sale. Ranchers are paid immediately when depositing plorts at the plort's current market value.
Plort market values fluctuate and some of the more savvy ranchers will hoard certain plorts for days when sale prices are high, especially rare plorts. - Slimepedia
Plorts are shot into the hole in of machine in order to sell them. In the process, they lose the plort but they gain newbucks in return. Prices of plorts fluctuate daily. This happens at 12 AM in-game time, when the plort market will be offline for a couple of in-game minutes. The fluctuation is based on how much Beatrix has sold with a little randomisation but they are always based on the plort's original value.